Some electronic components are susceptible to faulty operation in the presence of high levels of electromagnetic interference (EMI). EMI is any unwanted signal transmitted by electromagnetic induction or electromagnetic radiation that affects an electrical circuit. There are many potential sources of EMI such as digital clock signals in microprocessors and microcontrollers, periodic signals used in switched mode power supplies, local oscillator signals used in radio circuits to tune radio frequency (RF) signals, periodic noise from induction motors, and the like.
Several different standards bodies in different jurisdictions around the world define acceptable levels of generated EMI for a certified product. In order to reduce EMI below these standardized levels, circuit designers have sometimes used spread spectrum clock signals. Instead of having a constant frequency, spread spectrum clock signals have frequencies that vary over a certain range to reduce the radiated energy at any given frequency to below the standardized level. In order to efficiently implement spread spectrum, it is desirable to spread the energy of the clock signal as uniformly as possible over the desired range. One known technique to spread the spectrum over the desired range is to vary the frequency of the clock signal using a lower frequency triangular wave signal. While spreading the clock frequency using a triangular wave signal theoretically yields a perfectly uniform frequency spectrum, it becomes less than perfect when using real circuits such as phase locked loops (PLLs), limiting the effectiveness of the spread spectrum clock generator.
The use of the same reference symbols in different drawings indicates similar or identical items. Unless otherwise noted, the word “coupled” and its associated verb forms include both direct connection and indirect electrical connection by means known in the art, and unless otherwise noted any description of direct connection implies alternate embodiments using suitable forms of indirect electrical connection as well.